The present invention relates to the medical diagnostic arts. It finds particular application in connection with reducing the temperature at the forward bearing race of the bearing shaft of an x-ray tube rotor and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to dissipation of heat in other vacuum systems.
A high power x-ray tube typically includes a thermionic filament cathode and an anode which are encased in an evacuated envelope. A heating current, commonly on the order of 2-5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, on the order of 100-200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards an anode target area. The electron beam impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. The acceleration of electrons causes a tube or anode current on the order of 5-600 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot.
In high energy tubes, the anode rotates at high speeds during x-ray generation to spread the heat energy over a large area and inhibit the target area from overheating. The cathode and the envelope remain stationary. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam.
The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the glass envelope, and a rotor with an armature and a bearing shaft, within the envelope, which is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the armature which cause the armature and other portions of the rotor to rotate.
The temperature of the anode can be as high as 1,400xc2x0 C. Part of the heat is transferred to the rotor, including the armature and the bearing shaft. Heat travels through the bearing shaft to the bearing races and is transferred to the lubricated bearing balls in the races. The lubricants on the bearing balls become hot and tend to evaporate.
Because x-ray tubes operate in a vacuum requiring low vapor pressure materials, standard petroleum based lubricating compounds cannot be used. Thus, it is common in the industry to use solid metal lubricants, such as lead, on the bearing races. The evaporation of lead lubricant from a bearing race accelerates rapidly over 350xc2x0 C. These temperatures can be reached in the bearing, primarily during processing, and also during field life. The evaporation of lubricant leads to a rapid degradation of the bearing surfaces and premature tube failure. In an x-ray tube, the front bearing race is physically closer to the hot target than the rear bearing. Because of this, the front bearing runs about 100xe2x80x83 C. hotter than the rear bearing and fails at a much higher rate than the rear bearing.
To reduce lubricant evaporation, silver lubrication on the ball bearings is sometimes used in place of lead. Silver has a lower vapor pressure than lead and can be run at least 100xe2x80x83 C. hotter than lead. However, silver lubrication has a number of drawbacks. It tends to react with the bearing steel if it becomes too hot and causes grain boundary cracking and premature failure of the bearing. Additionally, silver requires more starting and running torque than lead, due to its lower lubricity. The torque imparts more residual heat into the bearing, through frictional and eddy current induction heating of the bearing and surrounding rotor body components. Silver lubricating material also creates more noise during operation than lead.
The present invention provides a new and improved x-ray tube and rotor and method of operation which overcome the above-referenced problems and others.
In accordance with one aspect of the present invention, a high energy x-ray tube for providing a beam of x-rays is provided. The tube includes an envelope which defines an evacuated chamber. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber and is struck by the electrons to generate x-rays. A rotor is provided for rotating the anode relative to the cathode. The rotor includes a rotor core having a high thermal conductivity such that heat is conducted by the core away from the anode, and a forward bearing race of a lower conductivity than the core, such that the core conducts heat past the forward bearing race. Lubricated bearings are received in the forward bearing race.
In accordance with another aspect of the present invention, a rotor for an x-ray tube is provided. The rotor includes a bearing member including a hollow cylindrical shaft formed from a material of a first thermal conductivity, the cylindrical shaft defining forward and rear bearing races on an exterior surface thereof to receive lubricated bearings therein. A neck is connected with the bearing member, adjacent the forward bearing races, to connect the rotor to an anode of the x-ray tube. An insert, received within the hollow shaft, and formed from a material of a second thermal conductivity, which is higher than the first thermal conductivity, transports heat away from the forward bearing race and reduces the temperature of the forward bearing race during operation of the x-ray tube.
In accordance with another aspect of the present invention, a method of reducing evaporation of a bearing lubricant in an x-ray tube having an anode and a rotor assembly connected therewith is provided. The rotor assembly includes a forward bearing race and a rear bearing race, the forward bearing race being closer to the anode than the rear bearing race. The method includes conducting heat around and past the forward bearing race toward the rear bearing race.
One advantage of the present invention resides in a reduction in operating temperature of the forward bearing of an x-ray tube bearing shaft.
Another advantage of the present invention is that the evaporation rate of the lubricant for the bearing balls is reduced.
Another advantage of the present invention is an increased life of the bearings and the tube.
Another advantage of the present invention is that it enables the use of lead as a bearing ball lubricant.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.